Vernier Go Direct GDX-POL Manual page 2

from Sensor Information. In LabQuest App, Identify is accessed by tapping the
sensor meter, then tapping Go Direct.
Using the Product
Follow these steps when using the Go Direct Polarimeter when performing
optical rotation experiments.
1. Connect the Go Direct Polarimeter according to the Getting Started
instructions.
2. Calibrate Go Direct Polarimeter.
a. Pour distilled water or the appropriate solvent in the Go Direct
Polarimeter cell to a height of 10 cm. It is important to read the height to
the nearest 0.1 cm. Read to the bottom of the meniscus.
b. Place the cell in Go Direct Polarimeter and select Finish Calibration.
c. When the polarimeter is ready, select Done.
3. You are now ready to add your optically active sample into the polarimeter
cell.
a. Rinse the polarimeter cell with a few milliliters of your optically active
sample. Pour your sample into the polarimeter cell to a height of 10.0 cm
(1.00 dm). It is important to read the height to the nearest 0.1 cm. Read
to the bottom of the meniscus.
b. Place the sample cell in the polarimeter.
c. Start data collection. Data collection will stop automatically. Data are
stored automatically in Instrumental Analysis. In LabQuest App, you can
store a run by tapping the file cabinet icon on the graph screen.
4. Record the first angle closest to 0° where the illumination is at a maximum.
This is the observed angle of rotation for the optically active sample (α).
There are several ways to locate this angle.
Statistics: To simply get the angle with the highest illumination,
l
highlight the peak of interest in Instrumental Analysis or LabQuest App.
In Instrumental Analysis, click or tap Graph Tools and select Statistics.
In LabQuest App, choose Statistics from the Analyze menu. Record the
angle value where the illumination is at a maximum, as presented in the
box. This method is the fastest and will result in reproducibility of the
angle of rotation measurement of ±2.0°.
Cosine Squared: To incorporate all your data into the fit, you can fit the
l
data to their true waveform, a cosine squared, in either Instrumental
Analysis or LabQuest App. In Instrumental Analysis, click or tap Graph
Tools and select Curve Fit. Then select cosine squared from the drop-
down list. In LabQuest App, choose Curve Fit from the Analyze menu.
From the list of available General Equations, select Cosine Squared. The
fit will run automatically. In this fit, the x-value corresponding to the
maximum y-value is obtained from the negative of the phase shift
parameter, –C. This is a nonlinear fit that undergoes numerous iterations
and may not converge, which may result in an unreasonable answer.
Make sure the resulting value is reasonable based on the data. This
method is the most time consuming; however, it will result in
reproducibility of the angle of rotation measurement of ±0.1°.
5. A compound will consistently have the same specific rotation under
identical experimental conditions. To determine the specific rotation of the
sample, use Biot's law:
where α is the observed optical rotation in units of degrees, [α] is the specific
rotation in units of degrees (the formal unit for specific rotation is degrees dm-1
mL g-1, but scientific literature uses just degrees), ℓ is the length of the cell in
units of dm, and c is the sample concentration in units of grams per milliliter.
6. To calculate a percent optical purity (or enantiomeric excess):
The value for the pure enantiomer can be obtained from literature or it can be
measured using a neat sample, considering the purity of that sample as described
by the manufacturer.
Calibrating the Sensor
You should perform a new calibration each time you power on the Go Direct
Polarimeter. Refer to the Using the Sensor section of this user manual for
additional instructions.
Specifications
Accuracy (optical rotation measurements)
Light source
Wavelength
Care and Maintenance
Battery Information
Go Direct Polarimeter contains a small lithium-ion battery. The system is
designed to consume very little power and not put heavy demands on the
battery. Although the battery is warranted for one year, the expected battery life
should be several years. Replacement batteries are available from Vernier
(order code: GDX-BAT-650).
Storage and Maintenance
To store Go Direct Polarimeter for extended periods of time, put the device in
sleep mode by holding the button down for at least three seconds. The red LED
will stop flashing to show that the unit is in sleep mode. Over several months,
the battery will discharge but will not be damaged. After such storage, charge
the device for a few hours, and the unit will be ready for use.
2
α = [α] ℓ c
±1°
LED
589 nm